Low layer count reflective polarizer with optimized gain

ABSTRACT

Multilayer reflecting polarizing films are disclosed having increased in-plane refractive index differences between adjacent microlayers along both the pass and block axis, and having negative refractive index differences between adjacent microlayers along the thickness or z-axis. Major front and back surfaces of the film exposed to air provide a Fresnel reflectivity component to the pass axis reflectivity, and the microlayers provide a microlayer component to the pass axis reflectivity, such microlayer component preferably having a reflectivity of p-polarized light that increases with incidence angle faster than the Fresnel reflectivity component decreases so as to substantially avoid off-axis gain peaks for p-polarized light. The films preferably utilize a relatively small total number of microlayers, arranged in a single coherent stack with monotonic optical repeat unit thickness profile, and at least some microlayers preferably include polyethylene naphthalate or a copolymer thereof.

RELATED APPLICATIONS

This is a continuation application of U.S. Ser. No. 12/935,500 filedDec. 20, 2010 which application is a national stage filing under 35U.S.C. 371 of PCT/US2009/038585, filed on Mar. 27, 2009, which claimspriority to U.S. Provisional Patent Application No. 61/040,910, filed onMar. 31, 2008, the disclosure of which is incorporated by reference inits entirety.

FIELD OF THE INVENTION

This invention relates generally to multilayer optical films, withparticular application to such films configured as polarizers that aresuitable for use in backlights for visual display systems.

BACKGROUND

Multilayer optical films, i.e., films that provide desirabletransmission and/or reflection properties at least partially by anarrangement of microlayers of differing refractive index, are known. Ithas been known to make such multilayer optical films by depositing asequence of inorganic materials in optically thin layers (“microlayers”)on a substrate in a vacuum chamber. Inorganic multilayer optical filmsare described, for example, in textbooks by H. A. Macleod, Thin-FilmOptical Filters, 2nd Ed., Macmillan Publishing Co. (1986) and by A.Thelen, Design of Optical Interference Filters, McGraw-Hill, Inc.(1989).

Multilayer optical films have also been demonstrated by coextrusion ofalternating polymer layers. See, e.g., U.S. Pat. No. 3,610,729 (Rogers),U.S. Pat. No. 4,446,305 (Rogers et al.), U.S. Pat. No. 4,540,623 (Im etal.), U.S. Pat. No. 5,448,404 (Schrenk et al.), and U.S. Pat. No.5,882,774 (Jonza et al.). In these polymeric multilayer optical films,polymer materials are used predominantly or exclusively in the makeup ofthe individual layers. Such films are compatible with high volumemanufacturing processes and can be made in large sheets and roll goods.

A multilayer optical film includes individual microlayers havingdifferent refractive index characteristics so that some light isreflected at interfaces between adjacent microlayers. The microlayersare sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference in orderto give the multilayer optical film the desired reflective ortransmissive properties. For multilayer optical films designed toreflect light at ultraviolet, visible, or near-infrared wavelengths,each microlayer generally has an optical thickness (a physical thicknessmultiplied by refractive index) of less than about 1 μm. Thicker layersare also typically included, such as skin layers at the outer surfacesof the multilayer optical film, or protective boundary layers (PBLs)disposed within the multilayer optical films, that separate coherentgroupings (referred to herein as “packets”) of microlayers.

For polarizing applications, e.g., reflective polarizers, at least someof the optical layers are formed using birefringent polymers, in whichthe polymer's index of refraction has differing values along orthogonalCartesian axes of the polymer. Generally, birefringent polymermicrolayers have their orthogonal Cartesian axes defined by the normalto the layer plane (z-axis), with the x-axis and y-axis lying within thelayer plane. Birefringent polymers can also be used in non-polarizingapplications.

In some cases, the microlayers have thicknesses and refractive indexvalues corresponding to a ¼-wave stack, i.e., arranged in optical repeatunits or unit cells each having two adjacent microlayers of equaloptical thickness (f-ratio=50%), such optical repeat unit beingeffective to reflect by constructive interference light whose wavelengthλ is twice the overall optical thickness of the optical repeat unit.Other layer arrangements, such as multilayer optical films having2-microlayer optical repeat units whose f-ratio is different from 50%,or films whose optical repeat units include more than two microlayers,are also known. These optical repeat unit designs can be configured toreduce or to increase certain higher-order reflections. See, e.g., U.S.Pat. No. 5,360,659 (Arends et al.) and U.S. Pat. No. 5,103,337 (Schrenket al.). Thickness gradients along a thickness axis of the film (e.g.,the z-axis) can be used to provide a widened reflection band, such as areflection band that extends over the entire human visible region andinto the near infrared so that as the band shifts to shorter wavelengthsat oblique incidence angles the microlayer stack continues to reflectover the entire visible spectrum. Thickness gradients tailored tosharpen band edges, i.e., the wavelength transition between highreflection and high transmission, are discussed in U.S. Pat. No.6,157,490 (Wheatley et al.).

Further details of multilayer optical films and related designs andconstructions are discussed in U.S. Pat. No. 5,882,774 (Jonza et al.)and U.S. Pat. No. 6,531,230 (Weber et al.), PCT Publications WO 95/17303(Ouderkirk et al.) and WO 99/39224 (Ouderkirk et al.), and thepublication entitled “Giant Birefringent Optics in Multilayer PolymerMirrors”, Science, Vol. 287, March 2000 (Weber et al.). The multilayeroptical films and related articles can include additional layers andcoatings selected for their optical, mechanical, and/or chemicalproperties. For example, a UV absorbing layer can be added at theincident side of the film to protect components from degradation causedby UV light. The multilayer optical films can be attached tomechanically reinforcing layers using a UV-curable acrylate adhesive orother suitable material. Such reinforcing layers may comprise polymerssuch as PET or polycarbonate, and may also include structured surfacesthat provide optical function such as light diffusion or collimation,e.g. by the use of beads or prisms. Additional layers and coatings canalso include scratch resistant layers, tear resistant layers, andstiffening agents. See e.g. U.S. Pat. No. 6,368,699 (Gilbert et al.).Methods and devices for making multilayer optical films are discussed inU.S. Pat. No. 6,783,349 (Neavin et al.).

FIG. 1 depicts one layer pair of a multilayer optical film 10. The film10 includes a large number of alternating microlayers 12, 14, only twoof which are shown for simplicity. The microlayers have differentrefractive index characteristics so that some light is reflected at theinterfaces between adjacent microlayers. The microlayers are thin enoughso that light reflected at a plurality of the interfaces undergoesconstructive or destructive interference to give the film the desiredreflective or transmissive properties. For optical films designed toreflect light at ultraviolet, visible, or near-infrared wavelengths,each microlayer generally has an optical thickness (i.e., a physicalthickness multiplied by refractive index) of less than about 1 μm.Thicker layers are also typically included, such as skin layers at theouter surfaces of the film, or protective boundary layers disposedwithin the film that separate packets of microlayers.

The reflective and transmissive properties of multilayer optical film 10are a function of the refractive indices of the respective microlayersand the thicknesses and thickness distribution of the microlayers. Eachmicrolayer can be characterized at least in localized positions in thefilm by in-plane refractive indices n_(x), n_(y), and a refractive indexn_(z) associated with a thickness axis of the film. These indicesrepresent the refractive index of the subject material for lightpolarized along mutually orthogonal x-, y-, and z-axes, respectively. InFIG. 1, these indices are labeled n1 x, n1 y, n1 z for layer 12, and n2x, n2 y, n2 z for layer 14, their respective layer-to-layer differencesbeing Δn_(x), Δn_(y), Δn_(z). For ease of explanation in the presentpatent application, unless otherwise specified, the x-, y-, and z-axesare assumed to be local Cartesian coordinates applicable to any point ofinterest on a multilayer optical film, in which the microlayers extendparallel to the x-y plane, and wherein the x-axis is oriented within theplane of the film to maximize the magnitude of Δn_(x). Hence, themagnitude of Δn_(y) can be equal to or less than—but not greaterthan—the magnitude of Δn_(x). Furthermore, the selection of whichmaterial layer to begin with in calculating the differences Δn_(x),Δn_(y), Δn_(z) is dictated by requiring that Δn_(x) be non-negative. Inother words, the refractive index differences between two layers formingan interface are Δn_(j)=n_(1j)−n_(2j), where j=x, y, or z and where thelayer designations 1,2 are chosen so that n_(1x)≧n_(2x), i.e., Δn_(x)≧0.

In practice, the refractive indices are controlled by judiciousmaterials selection and processing conditions. Film 10 is made byco-extrusion of a large number, e.g. tens or hundreds of layers of twoalternating polymers A, B, typically followed by passing the multilayerextrudate through one or more multiplication die, and then stretching orotherwise orienting the extrudate to form a final film. The resultingfilm is typically composed of many hundreds of individual microlayerswhose thicknesses and refractive indices are tailored to provide one ormore reflection bands in desired region(s) of the spectrum, such as inthe visible or near infrared. To achieve high reflectivities with areasonable number of layers, adjacent microlayers typically exhibit adifference in refractive index (Δn_(x)) for light polarized along thex-axis of at least 0.05. If the high reflectivity is desired for twoorthogonal polarizations, then the adjacent microlayers also can be madeto exhibit a difference in refractive index (Δn_(y)) for light polarizedalong the y-axis of at least 0.05.

The '774 (Jonza et al.) patent referenced above describes, among otherthings, how the refractive index difference (Δn_(z)) between adjacentmicrolayers for light polarized along the z-axis can be tailored toachieve desirable reflectivity properties for the p-polarizationcomponent of obliquely incident light. To maintain high reflectivity ofp-polarized light at oblique angles of incidence, the z-index mismatchΔn_(z) between microlayers can be controlled to be substantially lessthan the maximum in-plane refractive index difference Δn_(x), such thatΔn_(z)≦0.5*Δn_(x), or Δn_(z)≦0.25*Δn_(x). A zero or near zero magnitudez-index mismatch yields interfaces between microlayers whosereflectivity for p-polarized light is constant or near constant as afunction of incidence angle. Furthermore, the z-index mismatch Δn_(z)can be controlled to have the opposite polarity compared to the in-planeindex difference Δn_(x), i.e. Δn_(z)<0. This condition yields interfaceswhose reflectivity for p-polarized light increases with increasingangles of incidence, as is the case for s-polarized light.

The '774 (Jonza et al.) patent also discusses certain designconsiderations relating to multilayer optical films configured aspolarizers, referred to as multilayer reflecting or reflectivepolarizers. In many applications, the ideal reflecting polarizer hashigh reflectance along one axis (the “extinction” or “block” axis,corresponding to the x-direction) and zero reflectance along the otheraxis (the “transmission” or “pass” axis, corresponding to they-direction). If some reflectivity occurs along the transmission axis,the efficiency of the polarizer at off-normal angles may be reduced, andif the reflectivity is different for various wavelengths, color may beintroduced into the transmitted light. Furthermore, exact matching ofthe two y indices and the two z indices may not be possible in somemultilayer systems, and if the z-axis indices are not matched,introduction of a slight mismatch may be desired for in-plane indices n1y and n2 y. In particular, by arranging the y-index mismatch to have thesame sign as the z-index mismatch, a Brewster effect is produced at theinterfaces of the microlayers, to minimize off-axis reflectivity, andtherefore off-axis color, along the transmission axis of the multilayerreflecting polarizer.

Another design consideration discussed in '774 (Jonza et al.) relates tosurface reflections at the air interfaces of the multilayer reflectingpolarizer. Unless the polarizer is laminated on both sides to anexisting glass component or to another existing film with clear opticaladhesive, such surface reflections will reduce the transmission of lightof the desired polarization in the optical system. Thus, in some casesit may be useful to add an antireflection (AR) coating to the reflectingpolarizer.

Reflective polarizers are often used in visual display systems such asliquid crystal displays. These systems—now found in a wide variety ofelectronic devices such as mobile phones, computers, and some flat panelTVs—use a liquid crystal (LC) panel illuminated from behind with anextended area backlight. The reflective polarizer is placed over orotherwise incorporated into the backlight to transmit light of apolarization state useable by the LC panel from the backlight to the LCpanel. Light of an orthogonal polarization state, which is not useableby the LC panel, is reflected back into the backlight, where it caneventually be reflected back towards the LC panel and at least partiallyconverted to the useable polarization state, thus “recycling” light thatwould normally be lost, and increasing the resulting brightness andoverall efficiency of the display.

A representative visual display system 20 is shown in schematic sideview in FIG. 2. The system 20 includes an LC panel 22 and anillumination assembly or backlight 24 positioned to provide light to theLC panel 22. The LC panel 22 includes a layer of liquid crystal disposedbetween glass panel plates. The LC panel 22 is positioned between anupper absorbing polarizer 26 and a lower absorbing polarizer 28. Theabsorbing polarizers 26, 28 and the LC panel 22 in combination controlthe transmission of light from the backlight 24 through the displaysystem 20 to the viewer. Selective activation of different pixels of theliquid crystal layer by an electronic display controller results in thelight passing out of the display system 20 at the selected pixels, thusforming an image seen by the viewer.

The backlight 24 includes light sources, whether disposed in an edge-litconfiguration (light source 30 a) or a direct-lit configuration (lightsources 30 b), and distributes light from the sources over an outputarea that matches the viewable area of the LC panel 22. The lightsources may be cold cathode fluorescent lamps (CCFLs) or light emittingdiodes (LEDs), for example, and either individually or in combinationthey produce white light. The backlight 24 also includes a film stackgenerically depicted at 32, which may include various optical componentssuch as a diffuser plate, prismatic brightness enhancement film (BEF),and the multilayer reflective polarizer discussed above. The backlightincludes an enclosure whose inner bottom surface 34 a and inner sidesurfaces 34 b can be reflective to promote light recycling and enhancesystem efficiency. In some cases the backlight may also incorporate asolid light guide to transport light from edge-mounted light sources(light source 30 a) evenly over the output area.

In any case, the backlight provides an extended light source that the LCpanel 22 uses to produce an image that can be perceived by the viewer,who may be observing from on-axis (normal or near-normal) viewingdirections (viewer 36 a, positioned along the z-axis which isperpendicular to the multilayer reflective polarizer and to the otherextended optical components of the system 20), or from off-axis oroblique viewing directions (viewer 36 b).

One measure of performance of the reflective polarizer in the context ofa display system such as system 20 is referred to as “gain”. The gain ofa reflective polarizer or other optical film is a measure of how muchbrighter the display appears to the viewer with the optical filmcompared to the display without the optical film. More specifically, thegain of an optical film is the ratio of the luminance of the displaysystem (or of a portion thereof, such as the backlight) with the opticalfilm to the luminance of the display system without the optical film.Since luminance is in general a function of viewing orientation (seee.g. viewers 36 a, 36 b in FIG. 2), gain is also a function of viewingorientation. If gain is referred to without any indication oforientation, on-axis performance is ordinarily presumed. High gains arenormally associated with reflective polarizers that have very highreflectivity for the block axis and very high transmissivity (very lowreflectivity) for the pass axis, for both normally and obliquelyincident light. This is because a very high block axis reflectivitymaximizes the chance that a light ray of the non-useable polarizationwill be reflected back into the backlight so that it can be converted tothe useable polarization; and a very low pass axis reflectivitymaximizes the chance that a light ray of the useable polarization willpass out of the backlight towards the LC panel, with minimal loss.

Another performance measure of the reflective polarizer in the contextof a full RGB color display system is the amount of color the componentintroduces into the system, both on-axis and off-axis, as a result ofspectral non-uniformities in reflectance or transmission. Ideally, apolarizer reflects and transmits uniformly over the entire visiblespectrum from about 400 to 700 nm so that it introduces no significantperceived color into the display, either on-axis or off-axis. This ismost easily achieved if, again, the block axis reflectivity is as highas possible and the pass axis reflectivity is as small as possible, ormore precisely, if the portion of the pass axis reflectivity due tointerference effects from the microlayers is as small as possible. (Theremaining portion of the pass axis reflectivity, which is due to Fresnelsurface reflections at the front and back major surfaces of thepolymeric reflective polarizer exposed to air, has virtually no impacton color since such air-to-polymer surface reflections are substantiallyspectrally uniform.) Microlayer stacks that have neither very small norvery large reflectivities are more difficult to control for color overthe visible spectrum. This is because at intermediate reflectivities,even very small variations in the layer thickness profile of the stack,relative to an ideal or target thickness profile, can easily producespectral deviations from a target flat reflection spectrum that can bereadily perceived by the human eye in transmitted or reflected light.

In keeping with the above considerations, two commercially availablemultilayer reflective polarizer products, described in more detailbelow, are able to achieve good gain and low color characteristics usingfilm designs that are different in some respects but that both haveon-axis pass axis reflectivities that are very low by keeping Δn_(y)very small.

BRIEF SUMMARY

We have observed, however, that both of these commercial reflectivepolarizers exhibit off-axis gain peaks for p-polarized light. Theseoff-axis gain peaks are relatively small, but can detract from theon-axis gain or brightness to an extent that may be significant in someapplications. We have found that the gain peaks are related to the verysmall pass axis reflectivity component associated with the microlayers,in combination with Fresnel surface reflectivity associated with theouter surfaces of the polarizer and the dependence of that Fresnelreflectivity on incidence angle.

We therefore describe here, among other things, multilayer reflectivepolarizers that utilize new combinations of design features to provideexemplary gain and color performance while substantially avoiding theoff-axis gain peaks. We describe, for example, new selection criteriafor the polymer materials used in the reflective polarizer that increasethe in-plane index differences Δn_(x), Δn_(y) (while providing asuitable out-of-plane index difference Δn_(z)) to an extent that thepass axis reflectivity component associated with the microlayers, whilestill much smaller than the block axis reflectivity, is large enough toovercome the angular dependence of the Fresnel surface reflectivity ofthe outer surfaces so as to avoid the off-axis gain peaks. The selectioncriteria are also fortuitously compatible with low layer count films.

In exemplary embodiments, a reflective polarizer has a block (x) axisand a pass (y) axis, and first and second opposed major surfaces exposedto air and therefore exhibiting Brewster angle reflection minima, themajor surfaces being disposed perpendicular to a z-axis. A stack of Nmicrolayers is disposed between the major surfaces and arranged intopairs of adjacent microlayers that exhibit refractive index differencesalong the x-, y-, and z-axes of Δn_(x), Δn_(y), and Δn_(z) respectively,where Δn_(x)>Δn_(y)>0>Δn_(z).

In exemplary embodiments, the number N and the index difference Δn_(x)in combination are large enough to provide the polarizer with a highreflectivity for normally incident light polarized along the x-axis ofRblocknormal, Rblocknormal being at least 80%. The number N and theindex difference Δn_(y) in combination are small enough to provide thepolarizer with a low reflectivity for normally incident light polarizedalong the y-axis of Rpassnormal, Rpassnormal being 25% or less. Thenumber N and the index difference Δn_(y) in combination are large enoughso that the reflective polarizer exhibits a reflectivity greater thanRpassnormal for p-polarized light incident in the y-z plane at theBrewster angle of the first major surface. Preferably, Δn_(y) isresponsible for an incremental portion Rpassinc of Rpassnormal, and acorresponding portion of Δn_(x) equal to Δn_(y) is responsible for anincremental portion Rblockinc of Rblocknormal, and the number N is smallenough so that Rblockinc is comparable to Rpassinc. For example,Rblockinc is at least half of Rpassinc, or is at least equal toRpassinc.

In exemplary embodiments, the microlayers are arranged into opticalrepeat units each of which has an optical thickness, the optical repeatunits being arranged to provide a substantially monotonically orsmoothly increasing optical thickness profile. At least some of the Nmicrolayers comprise polyethylene naphthlate or a copolymer thereof, andN is 350 or less, or 300 or less, or in a range from 250 to 350, or in arange from 275 to 375. Alternatively, at least some of the N microlayerscomprise polyethylene terephthalate or a copolymer thereof, and N is 800or less, or 650 or less, or in a range from 300 to 650, or in a rangefrom 500 to 650. The reflective polarizer has a high reflectivityRblocknormal for normally incident light polarized along the x-axis, anda low reflectivity Rpassnormal for normally incident light polarizedalong the y-axis, Rblocknormal being at least 80%. Rpassnormal ispreferably less than 30% or 25% but is preferably at least 2% more thana combined normal incidence reflectivity of the major surfaces. Thereflective polarizer preferably exhibits a reflectivity greater thanRpassnormal for p-polarized light incident in the y-z plane at theBrewster angle of the first major surface.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a layer pair of a multilayer opticalfilm;

FIG. 2 is a schematic side view of a display system;

FIG. 3 is a perspective view of an optical film in relation to aCartesian coordinate system;

FIG. 4 is a graph of measured luminance versus polar angle ofobservation for a backlight in combination with various reflectivepolarizers, from which the angular dependence of gain can be discerned;

FIG. 5 is a graphical depiction of different combinations of refractiveindices for the alternating layers of a multilayer optical film;

FIG. 6 is a graph of modeled p-pol reflectivity as a function ofincidence angle for various multilayer film designs;

FIG. 7 is a graph of modeled on-axis reflectivity as a function ofnormalized in-plane refractive index difference for various multilayerfilm designs;

FIG. 8 is a graph that summarizes results of FIG. 7;

FIG. 9a is a graph of modeled on-axis gain as a function of the y-indexmismatch of various multilayer film designs; and

FIG. 9b is a graph of modeled hemispheric gain as a function of they-index mismatch of the various multilayer film designs.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In order to more clearly describe the off-axis gain behavior observed incommercially available reflective polarizers, we provide in FIG. 3 aperspective view of an optical film 40 in relation to a global x-y-zCartesian coordinate system. The film 40 may be a multilayer reflectivepolarizer, in which case the x-, y-, and z-axes can be identical to thelocal x-, y-, and z-axes discussed above. Alternatively, the film 40 maybe another optical film or surface, e.g., the front surface of a displaysystem. As shown, film 40 is laid flat, parallel to the x-y plane. Thefilm 40 has opposed major surfaces 40 a, 40 b exposed to air anddisposed perpendicular to the z-axis.

In reference to traditional polarizing films, light can be considered tobe polarized in two orthogonal planes, where the electric vector of thelight, which is transverse to the propagation direction of the light,lies within a particular plane of polarization. In turn, thepolarization state of a given light ray can be resolved into twoorthogonal polarization states: p-polarized and s-polarized light.P-polarized (“p-pol”) light is light that is polarized in the plane ofincidence, the plane of incidence being a plane containing both thelocal surface normal vector and the light ray propagation direction orvector. FIG. 3 illustrates a light ray 42 that is incident on oremerging from optical film 40 at an oblique angle θ relative to thesurface normal (z-axis), thereby forming a “plane of incidence” 44. (Forlack of an alternative term, “plane of incidence” will be used herein torefer to the plane containing the surface normal direction and the lightpropagation direction, both in cases where the light is incident on thefilm, and in cases where light is not incident on the film but insteadis emerging from the film. Likewise, “incidence angle” may be used torefer to the angle between the surface normal direction and the lightpropagation direction, both for light incident on the film and for lightemerging from the film.) If the film 40 is a polarizer, it includes apass axis 46 parallel to the y-axis and a block axis 48 parallel to thex-axis. The plane of incidence 44 of ray 42 is parallel to the blockaxis 48. Ray 42 has a p-polarized component that is in the plane ofincidence 44, and an s-polarized component that is orthogonal to theplane of incidence 44. The p-pol component of ray 42 is perpendicular tothe pass axis 46 and partially aligned with the block axis 48, while thes-polarized (“s-pol”) component of ray 42 is parallel to the pass axis46. FIG. 3 also shows another light ray 50 that is incident on oremerging from optical film 40 at the same oblique angle θ but in a planeof incidence 52 that is parallel to the pass axis 46. In this case, thep-pol component of ray 50 is perpendicular to the block axis 48 andpartially aligned with the pass axis 46, while the s-pol component ofray 50 is parallel to the block axis 48.

From this, one can see that depending on the direction of incidence,p-polarized light can be perpendicular to the pass axis in some casesand perpendicular to the block axis in others, and s-polarized light canbe parallel to the pass axis in some cases and parallel to the blockaxis in others. (Any arbitrary plane of incidence can be resolved intothe component incidence planes 44, 52.) Thus, a complete discussion ofthe behavior of s- or p-polarized light for anisotropic systems shouldinclude not only the angle of incidence (or e.g. the angle of emergenceor angle of observation) of the light, but also the plane of incidence(or e.g. the plane of emergence or plane of observation) of the light.

The gain of two known multilayer reflective polarizer products forp-polarized light was measured, and other characteristics were observed.

The first product, referred to herein as RP1, utilizes polyethylenenaphthalate (“PEN”) for one of the polymers and a copolymer or blendbased upon naphthalene dicarboxylic acid (“coPEN”), in particular a55/45 copolymer blend that included hexane diol to avoidcrystallization, for the other polymer. These polymers are coextruded inan alternating layer arrangement having 275 total layers, and theextrudate is sent through a 1×3 layer multiplier that divides theextrudate and stacks the three extrudate components atop each other, theresult being further processed and stretched to produce a finishedreflective polarizing film with 825 total microlayers separated intothree distinct microlayer packets (275 layers each) with optically thickprotective boundary layers (PBLs) therebetween, and optically thick skinlayers at the outer major surfaces. In the finished film, themicrolayers composed of PEN are birefringent while the microlayerscomposed of coPEN are substantially isotropic.

The second product, referred to herein as RP2, utilizes a 90/10copolymer blend based on naphthalene dicarboxylic acid (“coPEN”) for oneof the polymers and a copolyester (SA115, available from EastmanChemical Co.) for the other polymer. These polymers are coextruded in analternating layer arrangement with 275 total layers, and the extrudateis further processed and stretched with a parabolic tenter to produce afinished reflective polarizing film with 275 total microlayers arrangedin a single microlayer packet with optically thick skin layers at theouter major surfaces. In the finished film, the microlayers composed ofcoPEN are birefringent while the microlayers composed of copolyester aresubstantially isotropic.

Optical properties of these products are approximately as follows:

RP1 RP2 n1x 1.80 1.82 n1y 1.621 1.57 n1z 1.56 1.555 n2x, n2y, n2z 1.6121.57 Δn_(x) 0.188 0.256 Δn_(y) 0.009 (greater than 0 but less than 0.01)Δn_(z) −0.052 −0.015 N 825 275 Rpassnormal 12% 10% RpassnormalFresnel11% 10% Rblocknormal 94% 98%

In this table, n1 x, n1 y, n1 z, n2 x, n2 y, n2 z, Δn_(x), Δn_(y), andΔn_(z) are as described above. The value “N” is the total number ofmicrolayers in the polarizer. Rpassnormal is the average reflectivity ofthe film (including both reflectivity from the front and backair/polymer interfaces and reflectivity from the microlayers) overvisible wavelengths, e.g., from 400 to 700 nm, for light normallyincident on the polarizer and polarized along the pass (y) axis.RpassnormalFresnel is the portion of Rpassnormal attributable to theFresnel reflectivity of the front and back major surfaces exposed toair. Rblocknormal is the average reflectivity over visible wavelengths,e.g., from 400 to 700 nm, for light normally incident on the polarizerand polarized along the block (x) axis.

Note that RP2 achieves a higher block axis index difference (Δn_(x))than RP1 through a combination of a higher birefringent index n1 x and alower isotropic index n2 x. A higher Δn_(x) allows fewer layers to beused for the same block axis reflectivity, with other factors beingequal, since normal incidence reflective power increases quadraticallywith the respective in-plane refractive index difference. Note also thatRP1 and RP2 both have very low pass axis index differences (Δn_(y)), andlow normal incidence pass axis reflectivities. Lower still, however, isthe component of the normal incidence pass axis reflectivitiesattributable to the microlayers, which equalsRpassnormal−RpassnormalFresnel, or about 1% for RP1 and 0% for RP2.

We have investigated the gain characteristics of these reflectivepolarizing products. Specifically, we investigated the gain as afunction of angle for p-pol light in the y-z plane, i.e., the planecontaining the pass axis and the surface normal (see plane 52 in FIG.3). The films were placed on top of a diffusely transmissive hollowlight box (a backlight). The diffuse transmission and reflection of thelight box can be described as Lambertian. The hollow light box had adiffuse reflectivity of ˜83%. The box was illuminated from within usinga stabilized broadband light source. A standard linear absorbingpolarizer (i.e. an analyzer) was placed between the sample box and thecamera detector. The camera detector system was a conoscope made byAutronic-Melchers GmbH (Karlsruhe, Germany). Initially, after allowingthe backlight output to stabilize, the luminance of the backlight byitself was measured over a range of observation angles in the horizontalplane. This is the plane that contains the surface normal and thep-polarized pass axis light of the output polarizer, as with plane 52 ofFIG. 3. The result is plotted as luminance curve 60 in FIG. 4, where thehorizontal axis of the graph is the polar angle in degrees from thesurface normal (θ in FIG. 3), and the vertical axis is the measuredluminance in nits (nt). As can be seen, the luminance was relativelysymmetric with respect to the surface normal, and was relativelyconstant with angle until θ reached about ±50 degrees, beyond which adrop in luminance was observed.

A flat sheet of RP1 was then placed between the backlight and theanalyzer with the pass axis of RP1 aligned with the pass axis of theanalyzer, and the measurement repeated. The result is plotted asluminance curve 62. Clearly, the RP1 polarizer increases the luminanceof the system greatly over a wide range of observation angles, henceproviding significant gain. A secondary feature to note is the gradualrise in luminance with increasing angle out to about ±50 degrees,followed by a drop in luminance for more oblique angles. Recalling thatgain is given by the ratio of the luminance of the system with the film(curve 62) to the luminance of the system without the film (curve 60),the reader will readily appreciate that this feature corresponds to again peak at the polar angles of about ±50 degrees.

The sheet of RP1 was then removed and replaced with a flat sheet of RP2,oriented in the same manner as RP1, and the measurement repeated. Theresult is plotted as luminance curve 64. Note again the overallluminance increase relative to the backlight alone. Note also thegradual rise in luminance with increasing angle out to about ±50 to 60degrees, and a drop in luminance for more oblique angles. Similar toRP1, the polarizer RP2 can also be seen to exhibit a gain peak at thepolar angles of about ±50 to 60 degrees by comparing curve 64 to curve60.

The off-axis gain peaks of the RP1 and RP2 polarizers are due toBrewster angle effects at the outer air/polymer interfaces at the frontand back major surfaces of the polarizers. With no anti-reflectioncoating on the outer surfaces, each of the two air/polymer interfacescontributes a normal incidence (θ=0) reflectivity of about

${R_{0} = {\left( \frac{n_{p} - 1}{n_{p} + 1} \right)^{2} \times 100\%}},$where n_(p) is the refractive index of the outermost polymer layer(normally one of the materials used in the microlayers) and therefractive index of air is 1. For non-normal incidence, the reflectivityfor p-polarized light is close to but less than Ro at small angles θ,decreasing steadily with increasing angle θ until at the Brewster angleθ_(B) the p-pol reflectivity is zero. As the incidence angle θ continuesto increase beyond θ_(B), the p-pol reflectivity increases rapidly withincreasing θ.

The off-axis gain peaks of RP1 and RP2 are thus seen to be a consequenceof the fact that the pass axis of these polarizers becomes moretransmissive (less reflective) in p-polarized light with increasingincidence angle from θ=0 to θ_(B) due to Brewster angle effects of theouter air/polymer interfaces. This occurs because the two outerair/polymer interfaces are the primary contributor to the pass axisreflectivity at these incidence angles. The other contributor to thepass axis reflectivity over these angles is the plurality of microlayerswithin the multilayer film, whose contribution to reflectivity issecondary because Δn_(y) is so small. Of course, Δn_(y) is small for avery good reason—to minimize the pass axis reflectivity and therebymaximize the pass axis throughput.

Regardless of the cause of the off-axis gain peaks, they can beundesirable in applications that call for maximum on-axis gain becausethey bias the luminance and gain away from the normal.

We have found that it is possible to substantially eliminate theoff-axis gain peaks and increase the on-axis gain while also maintaininglow color, all in a simple one-packet film construction, by judiciousmaterials selection, processing, and film design. In exemplaryembodiments we achieve this by (1) increasing the pass axis indexdifference Δn_(y) and the block axis index difference Δn_(x) whilemaintaining a negative Δn_(z) by, for example, selecting a lowerrefractive index isotropic material, and (2) preferably limiting thefilm to a relatively small number of microlayers in a single packetconstruction. These design features are discussed further below. Curve66 in FIG. 4 is the measured luminance for a reflective polarizing filmembodying these features, discussed in more detail below.

We choose to increase the pass axis reflectivity attributable to themicrolayers enough so that the reflectivity increase of the microlayers(for p-pol light) with increasing incidence angle compensates for thereflectivity decrease of the air/polymer interfaces (for p-pol light)with increasing incidence angle, so that the overall reflectivity of thepolarizer (for p-pol light) increases monotonically with increasingincidence angle, or so that the reflectivity (for p-pol light) of themicrolayers is at least Rpassnormal for light that is incident at theBrewster angle θ_(B) of one or both of the air/polymer interfaces, orthat the reflectivity (for p-pol light) of the microlayers increasesfaster than the combined Fresnel reflectivites of the major surfacesdecrease. In some cases these conditions can be replaced with arule-of-thumb that specifies that Rpassnormal is at least 2% more thanthe combined normal incidence reflectivity of the major surfaces, orthat the portion of Rpassnormal attributable to the microlayers is atleast 2%. In order for the p-pol pass axis reflectivity of themicrolayers to increase with increasing incidence angle, theout-of-plane index difference Δn_(z) should be negative and the in-planeindex difference Δn_(y) should be positive, but less than the block axisindex difference Δn_(x). These relationships can be summarized byΔn _(x) >Δn _(y)>0>Δn _(z)

Although we increase the pass axis reflectivity, we preferably do notincrease it indiscriminately. We wish to keep it low enough to maintainreasonably high pass axis throughput. In some cases, we may increase itonly to the extent necessary so that the p-pol reflectivity increase ofthe microlayers compensates for the p-pol reflectivity decrease of theair/polymer surfaces, as discussed above. In some cases, we mayestablish a rule-of-thumb that Rpassnormal is no more than 30%, or 25%,or 20%, or 15%. Thus, a balance can be established between increasingthe reflectivity enough to compensate for the Brewster angle effects ofthe outer surfaces and keeping the reflectivity low enough to maintain areasonably high pass axis throughput and a high on-axis gain.

FIG. 5 is provided to demonstrate one approach for increasing thein-plane index differences Δn_(x), Δn_(y). In the figure, axes are shownfor refractive indices n_(x), n_(y), and n_(z). The axes are separatedvertically for clarity but otherwise have the same scale, and arefractive index scale starting at 1.0 (air) is also provided forgeneral reference. Points 70, 72, 74 represent the refractive indices ofmicrolayers composed of the first polymer material, i.e., n1 x, n1 y, n1z, respectively. The first polymer material is plainly birefringent. Adashed vertical line labeled n2 represents the refractive index ofmicrolayers composed of the second polymer material. The intersection ofthat line with the n_(x), n_(y), and n_(z) axes yield intersectionpoints (not labeled) that represent n2 x, n2 y, n2 z, respectively, allequal to each other. The second polymer material is isotropic. Thecombination of points 70, 72, 74 and the line n2 represent a multilayerreflective film comprising alternating birefringent and isotropicmicrolayers. The first and second polymer materials have been selectedand the film has been processed so that the refractive index of theisotropic material matches the y-index of the birefringent material, andso thatΔn _(x) >Δn _(y)=0>Δn _(z).This combination represents a reflective polarizer with no reflectivityprovided by the microlayers at normal incidence for light polarizedalong the pass axis. Such a film will likely exhibit off-axis gain peaksbecause the pass axis reflectivity provided by the microlayers willlikely not offset the Brewster angle effects of the front and back majorsurfaces of the polarizer exposed to air.

We can increase the pass axis reflectivity by substituting anotherisotropic material for the original isotropic material. In doing so, wemake sure that the new isotropic material has a lower refractive indexthan the original, and preferably also that the refractive index n2′ ofthe new isotropic material is greater than n1 z (point 74) to maintain anegative Δn_(z), so that the new refractive index differences Δn_(x)′=n1x−n2′, Δn_(y)′=n1 y−n2′, and Δn_(z)′=n1 z−n2′ satisfy the relationΔn _(x) ′>Δn _(y)′>0>Δn _(z)′,where a prime on the parameters indicates the new isotropic material.The new isotropic material preferably of course has material propertiesthat enable it to be coextruded with the polymer material that willbecome birefringent after orientation.

Note that this technique for increasing the index difference in they-direction also has the effect of increasing the index differences inthe other directions by the same amount. Thus, not only is the originaly-index difference Δn_(y) (which equals zero and thus is not shown inFIG. 5) increased by Δn_(y)′ to yield a new y-index difference now equalto Δn_(y)′; also, the original x-index difference Δn_(x) is increased bythe same amount Δn_(y)′ to yield Δn_(x)′, and the original z-indexdifference Δn_(z) is increased by the same amount Δn_(y)′ to yield asmaller negative value Δn_(z)′. The final y-index difference Δn_(y)′ inthe new construction can in this way be associated with equal refractiveindex adjustments along all three axes. An added benefit of increasingthe x-index difference by the same amount as the y-index difference isincreasing the reflective power of the microlayers for the block axis,which can be used to reduce the number of layers required in themicrolayer stack for a given block axis target reflectivity. Thus, ourapproach to eliminating the off-axis gain peaks by increasing the passaxis reflectivity can also help achieve a film with lower overall layercount and simpler design.

Although FIG. 5 is described for a birefringent/isotropic materialcombination in which alternative isotropic materials are selected, thisis not meant to be limiting. For example, the in-plane indices can beincreased by keeping the same isotropic material but substituting adifferent birefringent material, or keeping the same birefringentmaterial but changing the processing conditions (stretch amount, stretchprofile, temperature, dwell time, and so forth). Still further,constructions that use two different birefringent materials for thefirst and second polymer materials are also possible.

Turning now to FIG. 6, we see there modeled reflection curves thatdemonstrate the technique of increasing the pass axis reflectivity byincreasing the reflectivity of the microlayers along the y-axis. Eachcurve is the calculated reflectivity for particular multilayerreflective polarizer constructions for p-polarized light incident in they-z plane (see plane 52 of FIG. 3) as a function of incidence angle inair (see θ in FIG. 3). Each modeled polarizer construction assumed Ntotal microlayers arranged in a single stack and exposed to air at theouter surface of the first and last microlayer. The N microlayers werearranged in an alternating arrangement of a first and second polymer,with adjacent pairs of the first and second polymer forming opticalrepeat units with an f-ratio of 50%. The optical repeat units assumed alinear optical thickness profile ranging from 200 nm for the first layerpair (corresponding to a normal incidence reflection peak at 400 nm) to462 nm for the last layer pair (corresponding to a normal incidencereflection peak at 925 nm). The modeled reflective polarizerconstructions, referred to herein as RP6.1, RP6.2, RP6.3, and RP6.4, hadthe following additional properties:

RP6.1 RP6.2 RP6.3 RP6.4 n1y 1.61 1.61 1.61 1.61 n1z 1.505 1.505 1.5051.505 n2 1.61 1.595 1.564 1.564 Δn_(y) 0 0.015 0.046 0.046 Δn_(z) −0.105−0.09 −0.059 −0.059 N 275 275 275 175The refractive indices in the x-direction have no effect on the modelingand are not listed. The birefringent refractive indices n1 y, n1 z thatwere used are representative of 90/10 coPEN oriented at ˜145° C. at astretch ratio of about 5:1 at a strain rate of about 5 m/min. Theisotropic refractive indices n2 that were used are representative ofcoPEN 55/45 (for RP6.1), a blend of 46% 90/10 coPEN and 54% PETG (forRP6.2), and PETG (for RP6.3 and 6.4).

Application of optical modeling software to the polarizer constructionsRP6.1-RP6.4 yielded the respective p-pol reflectivity curves 80, 82, 84,86 shown in FIG. 6. Inspection of the curves reveals a significantBrewster angle minimum in curve 80 at an incidence angle between 40 and50 degrees, which produce off-axis gain peaks. Curves 82 and 86 exhibitvery slight but almost nonexistent Brewster angle minima. Curve 84exhibits no Brewster angle minimum. In at least that construction, theincreasing reflectivity of the microlayers with incidence angleovercomes the decreasing reflectivity of the two air/polymer surfacereflections to yield a net polarizer reflectivity that increasesmonotonically with increasing incidence angle. Such a film thereforewould exhibit no off-axis gain peaks. Comparison of curves 84 and 86demonstrates the effect of changing the layer number N of microlayers.

The modeling result of curve 84 was confirmed by fabricating a filmhaving substantially the characteristics described above for the RP6.3construction. A 90/10 coPEN (the birefringent material in the finishedfilm) and PETG (the isotropic material in the finished film) werecoextruded using a 275 layer feedblock and film-making equipment similarto that described in U.S. Pat. No. 6,783,349 (Neavin et al.), exceptthat no layer multiplier device was used. The layer thickness profile ofthe 275 layers was controlled to substantially match a target monotonicoptical thickness profile using an axial rod heater disposed in thefeedblock, whose temperature profile was dynamically adjusted along itslength during coextrusion to maintain the target layer thickness profilewith little deviation. The finished polarizing film, referred to hereinas RP6.3A, included an optically thick skin layer composed of PETG atboth the front and back of the microlayer packet, the skin layersforming the outermost layers of the film exposed to air.

A sheet of the RP6.3A film was placed atop the backlight referred to inconnection with FIG. 4 in the same manner as films RP1 and RP2, and theresulting luminance was measured in the same way. The measured luminanceis shown in curve 66 of FIG. 4. Unlike curves 62 and 64, curve 66 has nooff-axis gain peaks and has a maximum gain at substantially normalincidence. Moreover, the normal incidence gain is greater for RP6.3Athan for the commercial products RP1 and RP2, despite the increasednormal incidence pass axis reflectivity. The RP6.3A film was alsoinspected for on-axis and off-axis color, and it was found to be withinacceptable limits due to the careful layer thickness control duringfabrication.

As mentioned above, the polarizer film RP6.3A was fabricated without theuse of a layer multiplier. Although layer multipliers can simplify thegeneration of a large number of optical layers, they may impart smalldistortions to each resultant packet of layers that are not identicalfor each packet. For this reason, any adjustment in the layer thicknessprofile of the layers generated in the feedblock is not the same foreach packet, i.e., all packets produced by the multiplier cannot besimultaneously optimized to produce a uniform smooth spectrum free ofspectral disruptions. Thus, an optimum profile and low transmissioncolor polarizer can be difficult to make using multi-packet filmsmanufactured using multipliers. If the number of layers in a singlepacket generated directly in a feedblock do not provide sufficientreflectivity, then two or more such films (fabricated without any layermultipliers) can be attached to increase the reflectivity. Note,however, that the reflectivity obtained by adhering two multilayerpackets together (the total number of microlayers in the packets beingN) with an optically thick adhesive, or other material, is lessdesirable than the reflectivity obtained by a single packet of Nmicrolayers, as demonstrated in FIG. 7 below. The physical separation ofthe two packets in the former design results in the incoherent summationof the individual reflectivities of the packets, even though each packetindividually is coherent. The single packet design can provide a higherblock axis reflectivity for a given pass axis reflectivity, or canprovide a lower pass axis reflectivity for a given block axisreflectivity, than the two packet design. Related discussion of filmfabrication techniques, including axial rod heater control, can be foundin WO/2008/144656, incorporated herein by reference.

Increasing the pass axis reflectivity by increasing Δn_(y) is counter tothe normal polarizer design rules of maximizing the throughput of thepass axis. It also presents challenges with respect to on-axis andoff-axis color. As mentioned earlier, at intermediate reflectivities,even very small variations in the layer thickness profile of the stack,relative to an ideal or target thickness profile, can easily producespectral deviations from a target flat reflection spectrum that can bereadily perceived as color by the human eye in transmitted or reflectedlight. The increased importance of layer thickness control leads us topreferred film designs that are compatible with fabrication methods thatavoid the use of layer multipliers, for the reasons given above. Withouta layer multiplier, the number of microlayers in the finished productsubstantially equals the number of layers that are coextruded from thefeedblock. Given practical limits to feedblock design, this in turnleads us to seek film constructions that can function with a relativelysmall total number (N) of microlayers. This also has the benefit offilms that are physically thin, which may be beneficial in certainapplications.

We therefore now turn our attention to the number of microlayers (N)used in the reflective polarizer design, and to the distribution ofthose microlayers within the film (e.g., single coherent packet, versusmultiple packets separated by optically thick protective boundarylayers). We have already seen in FIG. 6, by comparing curves 84 and 86,that N can be an important factor along with the y-index difference inwhether off-axis gain peaks are observed.

FIG. 7 plots calculated reflectivity at normal incidence for variousmodeled multilayer film designs as a function of the refractive indexdifference between alternating microlayers. The model is not concernedwith 2-dimensional film characteristics, and thus the refractive indicesn1, n2 used in the model can represent any in-plane refractive index ofthe alternating layers, whether the x-axis or the y-axis refractiveindices. The z-axis refractive indices are unimportant because they haveno effect on normal-incidence behavior. On-axis reflectivity is plottedon the vertical axis and normalized refractive index difference(n1−n2)/(n1+n2) is plotted on the horizontal axis.

Several stack designs were modeled. All designs were constrained toreflect in a wavelength band extending from 400 to 925 nm. A first stackdesign (“SD1”) used 550 total layers (N=550) that were arranged into twoequal packets of 275 microlayers separated by an optically thick indexmatching medium. A second stack design (“SD2”) used 375 total layers(N=375) arranged in a single coherent multilayer stack. A third stackdesign (“SD3”) used 275 total layers (N=275) arranged into a singlecoherent multilayer stack. A fourth stack design (“SD4”) used 175 totallayers (N=175) arranged into a single coherent multilayer stack. A fifthstack design (“SD5”) used two packets of 138 microlayers each (N=276),separated by an index matching optically thick medium. (Results for theSD5 design can be compared to results for the SD3 design to ascertainthe effect of arranging substantially the same number of microlayersinto a single coherent packet versus into two packets that areindividually coherent but mutually incoherent.) Each design incorporateda front and back surface of the overall construction exposed to air,producing Fresnel reflectivity. Each design also assumes a monotoniclayer thickness gradient tailored to produce a normal incidencereflection band from 400 to 925 nm. Dispersion and absorption wereneglected, and the calculated reflectivities represent averages from 400to 700 nm, and they also represent averages from 400 to 925 nm.

The refractive indices of these stack designs were then allowed to vary.Initially, n1 and n2 were both set equal to 1.610. The value n1 was thenincreased up to 1.82 and beyond, and the reflectivity calculated foreach layer design/refractive index combination. The curves 90, 92, 94,96, 98 are the calculated reflectivities for the film designs SD1, SD2,SD3, SD4, SD5 respectively. Small refractive index differences yieldsmall values of normalized index difference, representative of pass-axisbehavior and labeled as a “Pass” region in the figure. Larger refractiveindex differences yield larger values of normalized index difference,representative of block-axis behavior and labeled as a “Block” region. Anumber of specific material combinations are also represented on thegraph as individual points. The table below shows refractive indexdifferences and normalized refractive index values (pass axis and blockaxis) for various low index isotropic materials having index n2 whencombined with a high index birefringent 90/10 coPEN material (having n1x=1.82, n1 y=1.61, n1 z=1.505):

Normalized Normalized index index n2 Δn_(x) Δn_(y) Δn_(z) diff (pass)diff (block) 55:45 1.610 0.21 0 −0.105 0 0.061 coPEN 75:25 1.595 0.2250.015 −0.09 0.005 0.066 coP: PETG 50:50 1.585 0.235 0.025 −0.08 0.0080.069 coP: PETG PETG 1.564 0.256 0.046 −0.059 0.014 0.076 coPET-f 1.5400.28 0.07 −0.035 0.022 0.083These respective normalized refractive differences are labeled in FIG. 7to enable identification of the individual points on the respectivecurves representing these material combinations. For example, each ofthe curves 90-96 has an individual datapoint at a normalized indexdifference value ((n1−n2)/(n1+n2)) of 0.022, corresponding to the indexdifference along the pass axis for the material combination 90/10 coPENand coPET-f, and each of the curves 90-98 also has an individualdatapoint at a normalized index difference value of 0.083, correspondingto the index difference along the block axis for the same 90/10coPEN-coPET-f material combination.

FIG. 7 thus shows how normal incidence reflectivity increases withchanges in the in-plane index difference, for both the pass (y) andblock (x) axes, and for different microlayer stack designs. The highestslope for the pass axis increase in reflectivity occurs for SD1, the2-packet system with individual coherent packets of 275 layers laminatedand reflecting in a non-constructive interference arrangement. This2-packet construction also has the smallest increase in block axisreflectivity for a given in-plane index difference, similar to the 375layer coherent stack design of SD2. The smallest slope for the pass axisincrease in reflectivity occurs for SD4, and this stack design also hasthe highest increase in slope for the block axis, i.e., most improvementfor a given in-plane index difference.

Comparison of curves 94, 96, 98 is instructive with respect to thedesirability of distributing the available microlayers in a singlecoherent packet rather than separating them into multiple packets. Thepass axis reflectivity for curve 98 (two packets, total N=276) issubstantially the same as that for curve 94 (single packet, N=275), butthe block axis reflectivity for curve 98 is closer to that for curve 96(single packet, N=175) than to curve 94. Thus, given the same totalnumber of microlayers, a single packet design can provide a higher blockaxis reflectivity for a given pass axis reflectivity, or can provide alower pass axis reflectivity for a given block axis reflectivity, than atwo packet design.

Selection of PETG as the low index isotropic material has the effect ofincreasing the in-plane index differences (both pass axis and blockaxis) by 0.046 relative to a construction having a perfect index matchalong the pass axis (using 55/45 coPEN as the low index isotropicmaterial). These increased in-plane index differences produce a 10%increase in pass axis reflectivity and a 6.6% increase in block axisreflectivity for stack design SD3 (275 layers, single stack—curve 94),but they produce a smaller 6.5% increase in pass axis reflectivity and alarger 11.7% increase in block axis reflectivity for stack design SD4(175 layers, single stack—curve 96). The smaller increase in pass axisreflectivity is beneficial for a polarizing film in maintaining a higherpass axis throughput, and the larger increase in block axis reflectivityis beneficial in keeping the loss/leakage of useable polarization low.

FIG. 8 is a graph that summarizes the lessons of FIG. 7 in a similar butsimplified format, and that graphically depicts the parametersRpassnormal, Rblocknormal, Rpassinc, and Rblockinc Normal incidencereflectivity is plotted against in-plane index difference n1−n2, wheresmall values of the index difference represent the pass axis and largervalues represent the block axis. Two curves are shown, a lower curve 100and an upper curve 102, the features of which are intended todemonstrate general trends. The lower curve 100 can represent amicrolayer stack design with relatively fewer microlayers N than analternative stack design for curve 102, each of the stack designs beingsingle packet designs that reflect over the same wavelength band.Alternatively, lower curve 100 can represent a microlayer stack designhaving the same number of microlayers N as that of curve 102, but thestack for curve 100 has the microlayers configured as a single coherentpacket whereas the stack for curve 102 has the microlayers divided intotwo packets separated by an optically thick index matching material (andagain each of the stack designs have a thickness gradient causing themto reflect over the same wavelength band).

We select a polymer material combination and processing conditions thatproduce a pass axis refractive index difference of Δn_(y)′ and a blockaxis refractive index difference of Δn_(x)′. These values helpcontribute (along with Fresnel reflectivity of the front and backsurfaces of the reflective polarizer) to a pass axis reflectivity atnormal incidence of Rpassnormal, and a block axis reflectivity at normalincidence of Rblocknormal. In the figure, Rpassnormal and Rblocknormalare only labeled for the lower curve 100 to avoid confusion, butcorresponding datapoints are also shown for the upper curve 102. Thesereflectivities can be compared to corresponding reflectivities one wouldobtain for the same stack design if a different low index isotropicmaterial were used, one that would cause the pass axis refractive indexdifference to drop to zero (and that would cause the block axisrefractive index difference to drop by the same amount). Both Δn_(y)′and Δn_(x)′ are thus decreased by the amount Δn_(y)′, yielding a newΔn_(y) of zero and a smaller Δn_(x) as shown. The resulting newreflectivity for the pass axis is smaller than Rpassnormal by an amountRpassinc, and the resulting new reflectivity for the block axis issmaller than Rblocknormal by an amount Rblockinc. For the preferredstack design (curve 100), Rblockinc is comparable to Rpassinc. Forexample, Rblockinc may be at least half of Rpassinc, or Rblockinc may beat least equal to Rpassinc.

FIGS. 7 and 8 help to illustrate the physics of pass and block axisreflectivity increases and demonstrate that coherent multilayer stacksare advantageous, but it is also useful to calculate the expected gainfor the different cases in a typical high efficiency commercialbacklight. This was done and the results plotted in FIGS. 9a and 9b . Arecycling model was used in which all reflected light was assumed to berandomized in the backlight, both in polarization and in angle ofpropagation. This is a good approximation for backlights that areconstructed with voided polyester reflectors that are commonly used inexisting commercial backlights.

A number of film stacks were modeled, each one using the same high indexbirefringent 90/10 coPEN material mentioned above, having n1 x=1.82, n1y=1.61, and n1 z=1.505 when properly oriented. A baseline design forcomparison purposes used a low index isotropic polymer material of indexequal to 1.61 to drive the y-index mismatch Δn_(y) to zero. In themodel, we include this embodiment, but then we also model a range ofalternative embodiments for which the isotropic index ranges from 1.61to 1.51, and we calculate both the on-axis gain and the total(hemispheric integrated) gain for polarized light delivered to an LCDpanel. The model assumed an absorption loss of 1% for all the films, anda backlight cavity efficiency of 90% (10% average total loss for raysentering the backlight cavity).

The film stacks that were modeled were the stack designs SD1 (twopackets of 275 layers each), SD2 (one packet of 375 layers), SD3 (onepacket of 275 layers), SD4 (one packet of 175 layers), and a packetdesign (“SD5”) having two packets of 138 microlayers each, separated byan index matching optically thick medium. The SD5 packet design, likethe others, included a layer thickness profile causing it to reflectover the wavelength band from 400 to 925 nm). Results for the SD5 designthus can be compared to results for the SD3 design to ascertain theeffect of arranging substantially the same number of microlayers into asingle coherent packet versus into two packets that are individuallycoherent but mutually incoherent.

The results are shown in FIGS. 9a (for modeled on-axis gain) and 9 b(for modeled hemispheric gain). The horizontal axis for each graph isthe pass axis refractive index difference Δn_(y), but it is to beunderstood that as Δn_(y) varies from 0 to 0.1, the block axisdifference varies in a corresponding fashion from 0.21 to 0.31, and theout-of-plane (z-axis) difference varies correspondingly from −0.105 to−0.005. In this regard, for convenience, one may express the results interms of an independent parameter ΔΔn that is added to or subtractedfrom each of the x, y, and z refractive index differences of aparticular baseline embodiment equally, where in this case the baselineembodiment for the respective stack designs may be those embodimentsusing the 90/10 coPEN material as the birefringent polymer and the 1.61index material as the isotropic polymer. Curves 110 a and 110 b are forstack design SD1, curves 112 a and 112 b are for stack design SD2,curves 114 a and 114 b are for stack design SD3, curves 116 a and 116 bare for stack design SD4, and curves 118 a and 118 b are for stackdesign SD5. Note that the gain per layer is higher for the single packetcoherent stacks compared to the 2-packet laminates of individualcoherent stacks. The overall maximum gain is generally achieved withhighest layer count, but this requires the greatest amount of PEN resincontent and complexity of the feedblock needed to control the layerprofiles to avoid perceived color. For each particular stack design, thegain is seen to reach a maximum at a certain value of Δn_(y) (or of ΔΔn)and then decrease. Preferably, Δn_(y) (or ΔΔn) is selected, viaappropriate material selection and processing conditions, to maximize orsubstantially maximize the gain for the stack design chosen. Forexample, if a maximum gain is achieved with a particular value of Δn_(y)or ΔΔn, then Δn_(y) or ΔΔn is preferably selected sufficiently close tothat particular value to achieve a gain that is at least 90% or 95% ofsuch maximum gain. In many of the stack designs, hemispheric or on-axisgain is maximized for Δn_(y) in a range from 0.01 to 0.06, or from 0.01to 0.05.

Thus, an optimum number of layers can be chosen to maximize gain withrespect to cost, for example. The optimum layer count for films having ahigh index layer having nx=1.82 is in the range from 150 to 400 layers,preferably in a range from 200 to 300, 250 to 350, or 275 to 325, or thelike, depending on customer expectations for cost and performance. Theoptimization procedure can also be applied to films having birefringentindices lower than those for PEN, e.g., to pure PET based birefringentmaterials. PET is generally lower cost than PEN, but requires morelayers due to the smaller index differences typically achievable andthus also slower line speeds. Accordingly, if some of the microlayerscomprise polyethylene terephthalate or a copolymer thereof, then N ispreferably 800 or less, or 650 or less, or in a range from 300 to 650,or in a range from 500 to 650.

Reflectivities of the foregoing films are generally reflectivityaverages over the visible spectrum, 400-700 nm, but other ranges canalso be used. Rpassnormal, Rblocknormal, Rpassinc, and Rblockinc, forexample, may thus represent reflectivity averages, but they may alsorepresent reflectivity averages that extend into the infrared region(e.g. 400-925 nm) to ensure adequate off-axis performance.

The reflectivity of the multilayer films may be difficult to measure atoblique angles, such as at a Brewster angle, especially if the film hassome surface structure or diffuser added to it. In those cases it issimpler to use the following procedure, using the well knownrelationship that R=1−T−A where A is the absorption and R and T aremeasured in an integrating sphere. Instruments such as the Perkin ElmerLambda 900 or Lambda 950 are suitable for this measurement. Firstdetermine A by measuring R_(norm) and T_(norm) at near normal incidence.Then measure T_(oblique) at the desired oblique angle such as at theair/polymer surface Brewster angle. The reflectivity then is given byR_(oblique)=1−T_(oblique)−A. The value of A may be slightly different atoblique angles and corrections may be made if desired. Near 55 degreeshowever the corrections are minor. Measurements of T_(oblique) aredifficult to measure with an integrating sphere if there is substantialdiffusion in the film. In addition, diffusers can increase theabsorption of a film. In order to remove measurement error due to thepresence of diffusers, a diffuse layer may be smoothed over by a coatingor by a laminate if it is a surface diffuser, or it may be stripped awaye.g. by polishing or by laser ablation if it is a bulk diffuserincorporated in an outer layer of the film such as a skin layer or aprotective boundary layer.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

The foregoing description is illustrative and is not intended to limitthe scope of the invention. Variations and modifications of theembodiments disclosed herein are possible, and practical alternatives toand equivalents of the various elements of the embodiments will beunderstood to those of ordinary skill in the art upon study of thispatent document. These and other variations and modifications of thedisclosed embodiments may be made without departing from the spirit andscope of the invention.

The invention claimed is:
 1. A reflective polarizer having a block axisand a pass axis corresponding respectively to an x-axis and a y-axis,the reflective polarizer comprising: first and second opposed majorsurfaces exposed to air and therefore exhibiting Brewster anglereflection minima, the major surfaces being disposed perpendicular to az-axis which is itself perpendicular to the x- and y-axes, the z-axisand the y-axis forming a y-z plane; and a stack of microlayers disposedbetween the major surfaces and arranged into pairs of adjacentmicrolayers that exhibit refractive index differences along the x-, y-,and z-axes of Δnx, Δny, and Δnz respectively, where Δnx>Δny>0>Δnz, thestack of microlayers having a total number N of said microlayers;wherein the number N and the index difference Δnx in combination arelarge enough to provide the polarizer with a high reflectivity fornormally incident light polarized along the x-axis, said highreflectivity referred to as Rblocknormal, said Rblocknormal being atleast 80%; wherein the number N and the index difference Δny incombination are small enough to provide the polarizer with a lowreflectivity for normally incident light polarized along the y-axis,said low reflectivity referred to as Rpassnormal, said Rpassnormal being25% or less; and wherein Δny is responsible for an incremental portionRpassinc of said Rpassnormal, and a corresponding portion of Δnx equalto Δny is responsible for an incremental portion Rblockinc of saidRblocknormal, and the number N is small enough so that said Rblockinc isat least half of Rpassinc, and wherein said Rpassnormal, Rpassinc,Rblocknormal, and Rblockinc are all averages over a same wavelengthrange.
 2. The polarizer of claim 1, wherein said Rblockinc is at leastequal to said Rpassinc.
 3. The polarizer of claim 1, wherein saidRblocknormal is at least 90%.
 4. The polarizer of claim 1, wherein saidRblocknormal is at least 95%.
 5. The polarizer of claim 1, wherein saidRpassnormal is 20% or less.
 6. The polarizer of claim 1, wherein saidRpassnormal is 15% or less.
 7. The polarizer of claim 1, wherein saidRpassnormal is at least 2% more than a combined normal incidencereflectivity of the major surfaces.
 8. The polarizer of claim 1, whereinthe reflective polarizer provides a gain when inserted into a backlightcavity, and wherein the gain is substantially maximized with respect toa parameter ΔΔn that can be used to increase or decrease the refractiveindex differences Δnx, Δny, Δnz equally.
 9. The polarizer of claim 8,wherein a particular value of the parameter ΔΔn achieves a maximum gain,and wherein the refractive index differences Δnx, Δny, Δnz provide again that is at least 90% of the maximum gain.
 10. The polarizer ofclaim 8, wherein the gain is an on-axis gain or a hemispheric gain. 11.The polarizer of claim 1, wherein the stack of N microlayers includesall microlayers disposed between the major surfaces.
 12. The polarizerof claim 1, wherein at least some of the microlayers comprisepolyethylene naphthalate or a copolymer thereof, and N is in a rangefrom 275 to
 325. 13. The polarizer of claim 1, wherein the number N andthe index difference Δny in combination are large enough so that thereflective polarizer exhibits a reflectivity for p-polarized lightincident in the y-z plane that monotonically increases with incidenceangle relative to the z-axis.
 14. The polarizer of claim 1, furthercomprising a mechanically reinforcing substrate attached to thepolarizer with an adhesive, the reinforcing substrate having astructured surface that provides light diffusion or collimation.
 15. Areflective polarizer having a block axis and a pass axis correspondingrespectively to an x-axis and a y-axis, the reflective polarizercomprising: first and second opposed major surfaces exposed to air andtherefore exhibiting Brewster angle reflection minima, the majorsurfaces being disposed perpendicular to a z-axis which is itselfperpendicular to the x- and y-axes, the z-axis and the y-axis forming ay-z plane; and a stack of microlayers disposed between the majorsurfaces and arranged into pairs of adjacent microlayers that exhibitrefractive index differences along the x-, y-, and z-axes of Δnx, Δny,and Δnz respectively, where Δnx>Δny>0>Δnz, the stack of microlayershaving a total number N of said microlayers, and wherein the microlayersare arranged into optical repeat units each of which has an opticalthickness, the optical repeat units being arranged to provide asubstantially monotonic optical thickness profile; wherein thereflective polarizer has a high reflectivity Rblocknormal for normallyincident light polarized along the x-axis, and a low reflectivityRpassnormal for normally incident light polarized along the y-axis, saidRblocknormal being at least 80%, and said Rpassnormal being less than25%; and wherein the reflective polarizer exhibits an averagereflectivity, over a wavelength range, greater than said Rpassnormal forp-polarized light incident in the y-z plane at the Brewster angle of thefirst major surface exposed to air, and wherein said Rblocknormal andRpassnormal are averages over the wavelength range.
 16. The polarizer ofclaim 15, wherein at least some of the N microlayers comprisepolyethylene naphthalate or a copolymer thereof, and N is 350 or less.17. The polarizer of claim 16, wherein N is 300 or less.
 18. Thepolarizer of claim 16, wherein N is in a range from 250 to
 350. 19. Thepolarizer of claim 16, wherein N is in a range from 275 to
 325. 20. Thepolarizer of claim 15, wherein at least some of the N microlayerscomprise polyethylene terephthalate or a copolymer thereof, and N is 800or less.
 21. The polarizer of claim 20, wherein N is 650 or less. 22.The polarizer of claim 20, wherein N is in a range from 300 to
 650. 23.The polarizer of claim 20, wherein N is in a range from 500 to
 650. 24.The polarizer of claim 15, wherein the stack of N microlayers includesall microlayers disposed between the major surfaces.
 25. The polarizerof claim 15, wherein Δnx is at least 0.2 and Δny is less than 0.05. 26.The polarizer of claim 15, wherein the major surfaces have a combinedreflectivity for p-polarized light incident in the y-z plane thatdecreases with increasing incidence angle and wherein the stack of Nmicrolayers has a reflectivity for said p-polarized light that increaseswith increasing incidence angle faster than the combined reflectivity ofthe major surfaces decreases, so as to provide the reflective polarizerwith a monotonic increase in reflectivity for p-polarized light incidentin the y-z plane.
 27. The polarizer of claim 15, further comprising amechanically reinforcing substrate attached to the polarizer with anadhesive, the reinforcing substrate having a structured surface thatprovides light diffusion or collimation.